Revisit the alkaline activation of peroxydisulfate and peroxymonosulfate

Many aspects of sulfate radical-advanced oxidation processes, particularly under alkaline medium, are largely debatable. Based on the available literature, the possible mechanisms of both activation of peroxydisulfate (PDS) and peroxymonosulfate (PMS) under alkaline conditions are reviewed. Thereby, several products (SO52-, HO2-) are formed presenting a complex interplay since they may be involved in persulfate activation and radical scavenging. Since both PMS and HO2- have dissociation centers, alkaline activation of persulfates is very strongly dependent on pH. Owing to some inconsistencies in the state-of-the-art literature, it seems that the activation of both PDS and PMS should be reconsidered in more detailed mechanistic studies. Relevant for alkaline activation, sulfate radical-based oxidation of hydroxide ions yielding hydroxyl radicals is not important for pH< 10.4 in chloride-poor waters (<<1.0 mM), while in the presence of chloride (e.g. 1.0 mM), hydroxyl radicals may be the most important reactive species.